Structural and Electronic Properties of Transition-Metal Oxides

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Structural and Electronic Properties of Transition Metal Oxides Attached to a Single-Walled CNT as a Lithium-Ion Battery Electrode: A First-Principles Study Liew Weng Tack, Mohd Asyadi Azam, and Raja Noor Amalina Raja Seman J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12904 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 22, 2017

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The Journal of Physical Chemistry

Structural and Electronic Properties of Transition Metal Oxides Attached to a Single-Walled CNT as A Lithium-Ion Battery Electrode: A FirstPrinciples Study

Liew Weng Tacka, Mohd Asyadi Azama,*, and Raja Noor Amalina Raja Semana a

Carbon Research Technology Research Group, Advanced Manufacturing Centre, Faculty of Manufacturing Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia

*Corresponding Email: [email protected] Tel: +606-331 6413 Fax: +606-331 6431

1 ACS Paragon Plus Environment


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ABSTRACT Single-walled carbon nanotubes (SWCNTs) and metal oxides (MOs), such as manganese (IV) oxide (MnO2), cobalt (II, III) oxide (Co3O4), and nickel (II) oxide (NiO) hybrid structures, have received great attention because of their promising application in lithium-ion batteries (LIBs). As electrode materials for LIBs, the structure of SWCNT/MOs provides high power density, good electrical conductivity, and excellent cyclic stability. In this work, first-principles calculations were used to investigate the structural and electronic properties of MOs attached to (5, 5) SWCNT and Li-ion adsorption to SWCNT/ metal oxide composites as electrode materials in LIBs. Emphasis was placed on the synergistic effects of the composite on the electrochemical performance of LIBs in terms of adsorption capabilities and charge transfer of Li-ions attached to (5, 5) SWCNT and metal oxides. Also, Li adsorption energy on SWCNTs and three different metal oxides (NiO, MnO2 and Co3O4) and the accompanying changes in the electronic properties, such as band structure, density of states (DOS) and charge distribution as a function of Li adsorption were calculated. Based on the calculation results, the top C atom was found to be the most stable position for the NiO and MnO2 attachment to SWCNT, while the Co3O4 molecule, the Co2+, was found to be the most stable attachment on SWCNT. The obtained results show that the addition of MOs to the SWCNT electrode enables an increase in specific surface area and improves the electronic conductivity and charge transfer of an LIB.

Keywords: metal oxides; SWCNT; lithium-ion battery; adsorption energy; first-principles calculation


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1. Introduction Carbon nanotubes (CNTs) have been intensively studied since their discovery by Sumio Iijima in 1991 as electrode material in Li-ion battery (LIB) applications. LIBs have been widely used in various applications, including mobile phones, laptops, and electronic devices [1]. Moreover, CNTs maximise the electrical conductivity, minimise solid-state path lengths for ion transport, and minimise path lengths for electron transport, which enables them to improve the power density of LIB. Their remarkable properties such as a wide high, surface area and good mechanical, electrical, and thermal properties make CNTs an ideal electrode material for LIB [2, 3, 4]. CNT can be synthesised by various methods, including laser ablation, arc-discharge, and chemical vapour deposition (CVD), in which CNT formation strongly depends on catalytic activity and catalyst lifetime [5]. CNTs are carbon allotropes with a cylindrical nanostructure comprising rolled-up graphene layers. Basically, singlewalled carbon nanotubes (SWCNTs) are made up of a single-cylinder graphite sheet by strong covalent bonding. Meanwhile, multi-walled carbon nanotubes (MWCNTs) are made up of several nested cylinders of graphite sheets, with an interlayer spacing of 0.34 to 0.36 nm and a diameter between 2 and 25 nm [6]. Metal oxides, such as manganese (IV) oxide (MnO2), cobalt (II, III) oxide (Co3O4), and nickel (II) oxide (NiO), are normally applied to the electroactive electrode material to improve the performance of electrodes in LIBs [7]. Most of the manganese dioxides, such as a-MnO2, g-MnO2 and l-MnO2, can accommodate significant lithium in their cavity, due to their large capacity and good ion insertion properties, and have been widely used in electrochemistry and photochemistry. For example, recent studies have shown that nanocomposite and mesoporous β-MnO2 exhibit a high capacity (320 mAh g-1) [8, 9]. NiO has a cubic structure, known as a rock salt structure, with octahedral nickel (II) and oxygen sites [10]. The NiO anode material in LIBs exhibits a higher capacity than graphite (372 mAh g-1) in terms of its theoretical capacity of 718 mAh g-1 [11], good rate capability and excellent cycle stability, which are believed to achieve high performance for future energy storage systems. In addition, Co3O4 is one of the most promising anode materials for potential graphitic carbon replacement [12]. In this study, therefore, the first-principles calculations based on density functional theory through DMol3 was used to investigate the electrochemical characteristics, such as adsorption capabilities and charge transfer of Li-ions attached to (5, 5) SWCNT and metal oxides. In this 3 ACS Paragon Plus Environment


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case, zigzag (5, 5) CNT is chosen since it has high stability metallic wave function. This causing the band opening or metal insulator transition. Metal-insulator transition contributes to the good electrical conductivity and makes it suitable to be use in lithium ion batteries. From previous study confirmed the trends of graphitic carbon structures having varying stability as a function of size. The energy of a graphene sheet and (10,0), (19,0), (5,5) and (10,10) SWCNTs as a function of the number of atoms in each system. For systems smaller than about 400 atoms with the smallest diameter, the (5,5) and (10,0) are the most stable. This is due to the smaller nanotubes are longer and thus have a smaller percentage of edge atoms than the larger nanotubes [13]. Using density functional theory (DFT), we calculated the Li adsorption energy on SWCNTs and three different metal oxides (NiO, MnO2 and Co3O4) and the accompanying changes in the electronic properties, such as band structure, density of states (DOS) and charge distribution as a function of Li adsorption. We also investigated the number of Li+ ions that can be inserted or removed from the SWCNTs/MO.


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2. Methods All the DFT calculations in this study were performed using a generalised gradient approximation (GGA) Perdew–Burke–Ernzerhof (PBE) function to treat the electron exchange–correlation energy of interaction electrons with a double numerical basis and a dpolarisation functional basis set [14]. GGA tend to underestimate band gaps due to selfinteraction errors. In quantum chemistry, self-interaction errors are also often largely responsible for the underestimation of chemical reaction barrier heights and of orbital energy gaps between highest occupied and lowest unoccupied molecular orbitals and for the overestimation of polarizabilities and hyperpolarizabilities of conjugated system [15]. GGA corrects the local density approximation (LDA) errors, such as dispersion forces between molecules. Compared with LDA, GGA effectively described the electronic subsystem, including binding energy and adsorption energy. The SCF tolerance and cycle are 1.0 × 10-6 and 1000, respectively. K-point sampling was set to 1 × 1 × 1. The isolated NiO molecule and pure armchair were fully optimised in rectangular simulation boxes of 25 × 25 × 15 Å and structurally optimised. The adsorption energy (Eads) is calculated according to this formula; Eads = Emetal oxides /CNT − (Emetal oxides + ECNT)

(1)

where E(CNT), E(metal oxides), and E(NiO/CNT) were denoted as the calculated energy of a pure CNT, the metal oxide molecule, and the total energy of a NiO/SWCNT unit cell adsorbed to the SWCNT, respectively. A negative value of Eads means that the adsorption of the NiO adsorbed is thermodynamically stable on its SWCNT substrate, which may be due to the decreased residual forces on the surface of the adsorbent. This causes a decrease in the surface energy of the adsorbent. Population analysis (Hirshfeld charge analysis) was defined relative to the deformation density (difference between the molecular and the unrelaxed atomic charge density). The quantum calculations were based on the linear combination of atomic orbital, geometric optimisation, and adsorption energy using the DMol3 package, while CASTEP code was used for study of electronic properties [16, 17].



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3. Results and discussion To obtain the configuration of the Li adsorption on the SWCNT/MO hybrid system, we fully optimized the geometry of the SWCNT/MO hybrid system before placing Li atoms on it. Three different MOs structures (MnO2, Co3O4 and NiO) were used to study Li-ion adsorption in SWCNT/MOs. Figure 1 shows the three possible potential adsorption sites for MOs on pristine (5, 5) SWCNT and its structure in a unit cell of SWCNT. Centroid of hexagon or hollow of carbon atom (Site-H), top of carbon atom (Site-T) and bridge of carbon atom (Site-B) are the possible positions of the MOs on SWCNT.

Figure 1: (a) Possible position of metal oxide attached on SWCNT, and (b) unit cell structure of the sites of metal oxides attachment on Pristine (5, 5) SWCNT

NiO Attached to (5, 5) SWCNT The lowest binding energy was calculated in order to determine the most stable adsorption site for NiO to the (5, 5) SWCNT. The binding energy, binding distance and surface area of SWCNT/NiO based on the interaction site are shown in Table 1.


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Table 1: The comparison of binding energy, binding distance and surface area at difference sites of SWCNT/NiO

Interaction Type

Binding energy (eV)

Binding distance(Å)

Surface area ( Å2)

Top /Ni-C

-3.2001

2.106

665.73

Bridge /Ni-C

-3.0749

1.984~2.141

665.66

Bridge/Parallax Ni-C

-3.1021

2.148~ 4.396

665.68

Centroid/Ni-C

-2.9116

2.026~3.197

665.51

Centroid/O-C

1.5402

2.295~2.540

665.52

Fig. 2a-e shows the optimised structure of Top/Ni-C, Bridge/Ni-C, Bridge/parallax NiC, Centroid /Ni-C and Centroid /O-C. From Fig. 2a, it can be seen that the top site of the pristine SWCNT showed the most stable position and better binding energy and binding strength compared to others, with the lowest binding energy of -3.2001 eV. Therefore, the top side was preferred to adsorb Ni atoms (NiO) compared to O atoms due to its strong electrostatic affinity for the locally positively charged Ni2+ species, and its unfilled d orbitals allow Ni2+ to favourably attach to the surface of SWCNT. Moreover, positive values of binding energy indicate that O atoms were repulsed on centroid pristine SWCNT, with the binding distance of 2.295–2.540 Å and further away from the centroid pristine SWCNT compared to Ni (2.026–3.197 Å). This is because O atoms and negatively charged SWCNTs repulse each other. Density functional negatively charged ions are unsteady because of the incorrect asymptotic behaviour of their function [18]. Note that the binding energy of the top Ni was almost similar with that of the other site. However, the top side has a larger surface area (665.73 Å2) compared to the bridge (665.66 Å2) and centroid (665.51 Å2) sides. The surface area played an important role in improving the performance of LIB. On the other hand, the binding distance of NiO was similar to the binding distance of the original distance (1.641Å). Thus, NiO much prefers to be attached to the top side of SWCNTs. NiO is a charge-transfer-type insulator. NiO displays a p-type conductivity with hole carriers generated by caption vacancies, and the valence configurations of NiO are 3d84s2 and 2s22p4 for Ni and O atoms, respectively [19]. On the other hand, Figs. 2f and g display the 7 ACS Paragon Plus Environment


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band structure of the NiO/SWCNT hybrid system. The Fermi level broadens and reduces the band gap due to the highly occupied valance band and unoccupied state in the conduction band close to the Fermi level. The overlapping shaded region indicates that the stability of the SWCNT/NiO was mainly supported by the hybridisation between the Ni-3d orbital and the SWCNT-p orbital. Moreover, there are additional bands on the valence band as compared to NiO and pristine SWCNT alone, which lower the band gap to 0 eV (Fig. 2g). Above all, the attachment of NiO on SWCNT can effectively improve the charge transfer, and the conductivity of the nanotube will change after the NiO adsorption. Hence, this SWCNT/NiO was the best approach to improve electrical conductivity.

Figure 2: The optimized structure of SWCNT/NiO represented by (a) Top/Ni-C (b) Bridge/Ni-C (c) Bridge/parallax Ni-C (d) Centroid /Ni-C (e) Centroid/O-C (f) the band structure and (g) density of state


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MnO2 Attached to (5, 5) SWCNT The comparison of binding energy, binding distance and surface area in three different sites of SWCNT/MnO2 is shown in Table 2. We also calculated the adsorption strength of the MnO2 adsorbed to the top, bridge and centroid adsorption sites. The top site (-2.88 eV) was found to be the most favourable, followed by the bridge site (-2.77 eV) and the centroid site (0.7102 eV), as shown in Fig. 3. These results may be attributed to the fact that MnO2 has two O atoms, which possess more negative charges and electronegativity to NiO. Therefore, the binding energy for MnO2 was less than that of NiO. For surface area, molecular MnO2 attached to the top side of SWCNTs has a larger surface area than NiO.

Table 2: The comparison of binding energy, binding distance and surface area at difference sites of SWCNT/MnO2

Interaction Type

Binding energy (eV)

Binding distance (Å)

Surface area (Å2)

Top / (Mn-C)

-2.8787

3.150

673.26

Bridge/ (Mn-C)

-2.7739

2.084~2.259

672.27

Centroid / (Mn-C)

-0.7102

2.075~2.622

667.86

The band structure for SWCNT/MnO2 displays an overlapping shaded region with a band gap value of 0 eV, indicating improvement in terms of electron excitation when MnO2 attaches to SWCNT (Fig. 3d). The binding distance of MnO2 is the closest to SWCNT, which is 3.150 Å, and it is more than that of NiO. The electronegativity of MnO2 is higher than that of NiO, and the weak binding energy of MnO2 increases exchange splitting of the Mn-d states and causes it to move away from the SWCNT. All MnO2 polymorphs share a simple atomic structure, with small Mn4+ ions in a spin-polarised 3d3 configuration and large, highly polarisable O2- ions in a spin-unpolarised 2p6 configuration [20]. From the electronic DOS, SWCNT/MnO2 display metallic behaviour (fixed density of states at EF), and this metallic state changes slightly with different stresses, as shown in Fig. 3e. Furthermore, an additional state near the Fermi level, which could be the reason for the lower band gap when MnO2 attached to SWCNT, can be clearly seen in graphs of DOS. The MnO2 acted as a 9 ACS Paragon Plus Environment


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semiconductor with the Fermi level inside the band gap, with a value of 1.33 eV [21]. The interaction of hybridisation between the Mn-3d orbital and the SWCN-p orbital can decrease to 0 eV in the MnO2 band gap, which may be attributed to the enhancement of the superior charge transport properties than to MnO2 alone.

Figure 3: The optimized structure of SWCNT/MnO2 represented by (a) Top/Mn-C (b) Bridge/Mn-C (c) Centroid/Mn-C (d) the band structure and (e) density of state

Co3O4 Attached to (5, 5) SWCNT. Table 3 shows that the Co3O4 molecule has the most stable configurations on SWNCT. The Co3O4 is preferentially located on a Co2+ site, followed by the Co3+ site, and then the O2- site of SWCNT. Co2+ ions are the active sites, and the catalytic ability of these ions is closely related to the density of catalytically active sites [22]. On the other hand, the value adsorption energy of Co2+ is more negative compared to Co3+. Co2+ has the closest binding distance to SWCNT, 2.076 Å (Fig. 4). The O2- adsorption site of the SWCNT has the weakest energy of approximately 1.53 eV. The binding energy is highest on the Co2+ position compared to the other positions. The negative sign indicates that the Co2+ atom adsorbed on SWCNT, having better binding energy and binding strength.


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Table 3: The comparison of binding energy, binding distance and surface area at difference sites of SWCNT/Co3O4 Interaction Type

Binding energy (eV)


Surface area (Å2)

(Co2+-C)

- 2.0245

2.076

706.37

(Co3+-C)

- 1.6789

2.259

703.74

(O2--C)

- 1.5320

2.622

705.35

The electronic DOS of a pure Co3O4 is Egap=1.60 eV [23]. Co3O4 is a p-type semiconductor with efficient electronic and magnetic properties (Fig. 4d, e). It is considered the most versatile oxide among the transition metal oxides and is widely applied in various fields [22]. The electronic DOS and band structure can be used to determine whether the interaction between Co3O4 with SWCNT will increase the conductivity performance of hybrids. The overlapping shaded region shows a plot of the 3s, 3p, and 3d orbitals of Co3O4 and 2s and 2p orbitals of the neighbouring C atom. The results obtained show that a decrease in the Co3O4 band gap to 0.091 eV should lead to a more significant enhancement of the superior charge transport properties than to Co3O4 alone because of the interaction of hybridisation between the Co-3d orbital and the SWCNT-p orbital. This result proves that the cobalt oxide, is converted into a more conductive form than that of the semiconductor cobalt oxide. Therefore, the electron transfer can likely ‘jump’ from the valence band to the conduction band. This result can accelerate the electron transfer between Li+ with O2-.The binding energy of Co3O4 is less than that of other MnO2 and NiO, but it is still engaged with the chemisorption mechanism. Thus, we can say that the mechanism of adsorption of Co3O4 is similar to that of MnO2 and NiO. However, it is with lower value since the binding energy is lower, especially when determining the stable position. Additionally, Co3O4 is the closest to SWCNT compared to MnO2 and NiO. Meanwhile, the surface area of Co3O4 (706.37 Å2) was greater than that of NiO (673.26 Å2) and MnO2 (673.26 Å2). As the surface area of electrodes increase, the efficiency of electrochemical reactions will enhance the ion exchange between electrodes and electrolytes. The charge transfer between CNTs and Li-ions will improve the interactions between metal oxides and SWCNTs.



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Figure 4: The optimized structure of SWCNT/Co3O4 represented by (a) Co2+-C (b) Co3+-C (c) O2--C (d) the band structure and (e) density of state Li-Ion Adsorption on Metal Oxides/SWCNT To better understand the fundamental mechanism of the Li-ion attached to the surface SWCNT/MO and the adsorption properties of Li adsorbed onto SWCNT/MO, all the possible the Li-ion atom adsorptions onto SWCNT/MOs of the system are shown in Fig. 5. These selected sites are as follows: first, the upside O atom which was above the MOs; second, the Site-B: between the metal with O atom; third, the downside metal atom. Based on the binding energy, the most stable site for the Li-ion to adsorb was determined.

Figure 5: Three possible sites of Li-ion attachment to SWCNT/metal oxides 12 ACS Paragon Plus Environment

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Due to the electron affinity, the positively charged Li-ion is highly attached to the negatively charged O atom. For NiO, Site-B is the most stable adsorption position in the SWCNT/NiO. However, Fig. 6 shows the before and after optimization of Li-ions attached on NiO/SWCNTs. The illustration shows that the Li-ion highly prefers attachment to the top side of the O atom. It can be assumed that the most stable position for the Li-ion is on the upside of the O atom. The Li-ion binding distance to O atoms is 1.653 Å.

Figure 6: Li-ion adsorption on site-B of SWCNT/NiO; (a) before optimization and (b) after optimization For MnO2, the upside of the O atom is the most stable position for the Li-ion upon adsorption on SWCNT/MnO2. As shown in Table 4, the Li-ion adsorption to any type of SWCNT/MO is considered the chemisorption mechanism. This is because the adsorption energy is very high, and is more than 2.52 eV. The Li-ion binding distance is closest to that of SWCNT/MnO2, at 1.784–2.895 Å (Fig. 7).

Figure 7: Li-ion adsorption on SWCNT/MnO2 at (a) site-U and (b) site-B



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Table 4: The comparison of stable position for Li-ions attachment on SWCNT/MOs

Type

Li-ion adsorption on SWCNT/NiO

Li-ion adsorption on SWCNT/MnO2

Li ion adsorption on SWCNT/Co3O4

Site

Binding energy (eV)


U

-3.1280

1.641

B

-3.1344

1.653

D

-

-

U

-2.5236

1.784~2.895

B

-2.5120

1.743

D

-

-

U

-3.6598

1.837~ 1.838

B

-3.4765

1.843~1.848

D

-

-

In the Co3O4 system, the adsorption energy was -3.6598 eV. Therefore, it was thermodynamically feasible, and the geometric structures were proven acceptable. The Li-ion was not suitable for attachment to the D site because electron affinity repulses the metal atom and leads to failed DFT calculations. The Li-ion atom adsorbed on the surface of Lewis base sites (O ions) over SWCNT/MO is shown in Fig. 8.

Figure 8: Li-ion adsorption on SWCNT/Co3O4 at site-U 14 ACS Paragon Plus Environment

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The Hirshfeld analysis of Li-ions on base acid sites is shown in Table 5. Electron charge transfer plays a primary role in the stability of an interacting system and electronic properties [24]. The adsorption configuration of base acid sites is that the Li atom connects with O atoms in the adsorption substrate. The Hirshfeld analysis of Li-ion proved the transfer of electrons from Li to the adsorption substrate. Thus, it improves the oxidation capacity of the adsorption substrate. Based on this result, Hirshfeld analysis of SWCNT/NiO, SWCNT/MnO2, and SWCNT/Co3O4 are 0.452e, 0.487e, and 0.434e for Li-ion atoms, respectively. It can be suggested that the higher the value of Hirshfeld analysis, the higher the reactivity. Therefore, the Li-ion on SWCNT/MnO2 loses electrons more easily than other SWCNT/Co3O4 and SWCNT/NiO. Based on the population analysis, SWCNT/MnO2 was the most effective in transferring charge. Table 5: Hirshfeld analysis for Li-ions attachment on SWCNT/MOs

Li-ion attachment on SWCNT

Li

Hirshfeld analysis (electron)

SWCNT/NiO

0.452e for Li

SWCNT/MnO2

0.487e for Li

SWCNT/Co3O4

0.434e for Li



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4. Conclusions In summary, the most stable atomic/molecule structure of pristine (5, 5) SWCNTs, metal oxides and Li-ions was successfully optimized. The total energy for NiO, MnO2, Co3O4, SWCNTs and Li-ions were 1583.2684 Ha, 1301.1739 Ha, 4448.5983 Ha, 4569.3754 Ha and 7.4512 Ha, respectively. The top C atom was found to be the most stable position for the NiO and MnO2 attachments on SWCNT with adsorption energies of -3.2001 eV and 2.8787 eV, respectively. For the Co3O4, Co2+ was found to form the most stable attachment on SWCNT with an adsorption energy of -2.0245 eV. For the electronic properties, when the NiO and MnO2 attached to the SWCNT, the band gap dropped immediately to 0 eV. The band gap of Co3O4 decreased from 0.43 eV to 0.091 eV after Co3O4 attachment to SWCNT. Decreases in the band gap contributed to the enhancement of the superior charge transport compared to metal oxides alone. Based on the population analysis, SWCNT/MnO2 is the most effective in transferring charge.

Acknowledgement The authors are grateful to the Universiti Teknikal Malaysia Melaka for the financial support and the UTeM Zamalah Scheme for the PhD support of R.N.A.R. Seman. Also, we offer special thanks to Dr. Abhijit Chatterjee (Materials Studio expert) and iMADE members of UiTM, Selangor, Malaysia for technically supporting this research.


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Figures and Tables Caption

Figure 1: (a) Possible position of metal oxide attached on SWCNT, and (b) unit cell structure of the sites of metal oxides attachment on Pristine (5, 5) SWCNT Figure 2: The optimized structure of SWCNT/NiO represented by (a) Top/Ni-C (b) Bridge/Ni-C (c) Bridge/parallax Ni-C (d) Centroid /Ni-C (e) Centroid/O-C (f) the band structure and (g) density of state Figure 3: The optimized structure of SWCNT/MnO2 represented by (a) Top/Mn-C (b) Bridge/Mn-C (c) Centroid/Mn-C (d) the band structure and (e) density of state Figure 4: The optimized structure of SWCNT/Co3O4 represented by (a) Co2+-C (b) Co3+-C (c) O2--C (d) the band structure and (e) density of state Figure 5: Three possible sites of Li-ion attachment to SWCNT/metal oxides Figure 6: Li-ion adsorption on site-B of SWCNT/NiO; (a) before optimization and (b) after optimization Figure 7: Li-ion adsorption on SWCNT/MnO2 at (a) site-U and (b) site-B Figure 8: Li-ion adsorption on SWCNT/Co3O4 at site-U

Table 1: The comparison of binding energy, binding distance and surface area at difference sites of SWCNT/NiO Table 2: The comparison of binding energy, binding distance and surface area in difference sites of SWCNT/MnO2 Table 3: The comparison of binding energy, binding distance and surface area at difference sites of SWCNT/Co3O4 Table 4: The comparison of stable position for Li-ions attachment on SWCNT/MOs Table 5: Hirshfeld analysis for Li-ions attachment on SWCNT/MOs


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Table of Contents graphic


Structural and Electronic Properties of Transition-Metal Oxides

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